1. Field of the Invention
The present invention relates to artificial spinal fusion implants to be placed across the intervertebral space left after the removal of a damaged spinal disc, and in particular to an improved, at least partially cylindrical, spinal fusion implant for implantation where two threaded cylindrical implants of requisite height would not fit within the transverse width of the spine.
2. Description of the Related Art
In the past, Cloward, Wilterberger, Crock, Viche, Bagby, Brantigan, Michelson and others have taught various methods involving the drilling of holes across the disc space between two adjacent vertebrae of the spine for the purpose of causing an interbody spinal fusion. Cloward taught placing a dowel of bone within that drilled hole for the purpose of bridging the defect and to be incorporated into the fusion. Viche taught the threading of that bone dowel. Bagby taught the placing of the bone graft into a metal bucket otherwise smooth on its surface, except for rows of radially placed holes communicative to the interior of the basket and to the bone graft. The Bagby device was disclosed as capable of being used in a horse. Brantigan taught the use of inert blocks preferably made of metal and having that metal at its external surface imitate the porosity of bone. Brantigan theorized that the bone dowel could be replaced entirely with a metal plug, that, while not itself active in the fusion, would nevertheless serve to support the vertebrae from within the disc space while allowing fusion to occur around it.
U.S. Pat. No. 3,844,601 issued to Ma et al. on Nov. 19, 1974, teaches a method and instrumentation for preparing rectangular spaces across the disc space into the adjacent vertebrae and for preparing a rectangular graft of the bone itself that is inserted in the rectangular spaces.
U.S. Pat. No. 4,743,256 issued to Brantigan on May 10, 1988 teaches the use of an inert artificial spacer in the shape of a rectangle in place of using a rectangular bone graft as taught by Ma et al.
U.S. Pat. No. 4,878,915 issued to Brantigan on Nov. 7, 1989, teaches the use of fully cylindrical inert implants for use in interbody spinal fusion. Such implants do not participate in the bone fusion process but act as inert spacers and allow for the growth of bone to the outer surfaces of the implants.
U.S. Pat. No. 4,834,757 issued to Brantigan on May 30, 1989, teaches a rectangular shaped, hollow spinal fusion implant for use in lieu of a rectangular bone graft or Brantigan""s earlier artificial inert spacer.
U.S. Pat. No. 5,015,247 issued to Michelson on May 14, 1991, teaches the use of a thin-walled, highly perforated, threaded, hollow cylindrical implant closed or closable at both ends, so as to be compressably loaded with bone or other fusion promoting materials. Additionally, the Michelson device may then be coated with a bone production inducing chemical such as hydroxyapatite. The Michelson patent, also discloses an improved method of drilling holes across the disc space and into the two adjacent vertebrae and safely installing cylindrical implants such that the entire surgical procedure may be conducted through a hollow cylindrical tube. The hollow cylindrical tube may be left in place throughout the surgical procedure and serves to hold the adjacent vertebrae in place relative to each other, permits the guarded drilling of the holes across the disc space, and permits the insertion of the implant through that same tube into the hole drilled across the disc space and into the adjacent vertebrae.
As regards this method of performing interbody spinal fusion using essentially cylindrical threaded implants, a special problem arises (see FIG. 1) when an attempt is made to place two cylindrical implants (considered to be the preferred number as it is a much more stable construct and has more surface area than a single implant placed centrally) side-by-side across a disc space and into the two adjacent vertebrae where the height of the disc space is such that it requires an implant of a diameter so large to penetrate into and significantly engage each of the adjacent vertebrae that it is no longer possible to place two such implants side-by-side and to still have them contained within the transverse width of the spine. If an attempt is made to remedy the problem by using smaller diameter implants placed side-by-side such that both would then be able to fit within the transverse width of the spine, then the implants would be of insufficient height to adequately engage the bone. If an attempt is made to remedy the problem by abandoning the side-by-side double implant construct in favor of a single, centrally placed implant, then where the implant is sufficiently large enough to occupy a sufficient portion of the transverse width of the disc space to promote firm stability, its vertical height and excursion into the vertebrae would be so severe that if any two consecutive disc spaces were to be operated upon, the vertebrae in between would be cut in half.
U.S. Pat. No. 5,055,104 issued to Ray on Oct. 8, 1991 (xe2x80x9cRay Patentxe2x80x9d) discloses an implant comprising a helical coil without wall members that is assembled after the coils are placed in the disc space between the vertebrae, which supposedly can then be removed after the vertebrae have become fused together. The Ray implant is defective and unworkable in that it is incapable of being used in the manner in which it is described as it is not possible to insert into hard bone an isolated helical coil without any wall members to support such a coil, which coil would be analogous structurally to a slinky. (See Ray Patent, FIGS. 1 and 7). Further, the Ray implant is unduly complex, because it would require the difficult, if not impossible, task of assembly within the disc space. FIG. 3 of the Ray Patent clearly reveals that Ray does not teach the use of a truncated cylindrical implant, but merely teaches the use of a truncated, helical coil appearing as a sharpened spring totally lacking any wall member which could be considered cylindrical. Therefore, Ray teaches only the use of an isolated thread which can only be inserted by rotation and cannot be linearly advanced.
If the overwhelming obstacles of the impossibility of inserting an isolated thread without wall members and the problem of the assembly within the disc space could be overcome, then the Ray implant, would still be unsafe for its intended purpose as it would be at high risk of spontaneous disassembly and mechanical failure. Further, there would be insufficient room to safely rotate such a device for insertion as it is the very lack of such room that requires the use of a device having a decreased transverse width.
There is therefore, the need for a spinal fusion implant that is capable of being inserted into a hole drilled across the disc space between two adjacent vertebrae and partially into the two adjacent vertebrae such that the spinal fusion implant is capable of fitting within the transverse width of the spine when placed side-by-side next to a second of its kind.
The present invention is an improved interbody spinal fusion implant that is capable of being inserted into a hole drilled across the disc space between two adjacent vertebrae and into the two adjacent vertebrae such that the spinal fusion implant is capable of fitting within the transverse width of the spine when placed side-by-side next to a second of its kind. The spinal fusion implant of the present invention comprises a thin-wall, multi-perforate, cylinder or partial cylinder, made of material appropriate for human implantation and having preferably, but not necessarily, one closed end and one end capable of being closed, such that an internal chamber can be filled and hold any natural or artificial osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material. The spinal fusion implant of the present invention relies on roughenings of the outer surface to enhance its stability. Depending on the dimension of the transverse width of the spine in which the spinal fusion implant is being inserted, the spinal fusion implant of the present invention may have one or more flat sides to reduce the width of the spinal fusion implant. The spinal fusion implant of the present invention incorporates at its rear end, an engagement means to facilitate insertion or extraction of the implant, preferably at its rear end. The implant of the present invention may be made of, filled with and/or coated with fusion promoting substances. Further, the spinal fusion implant of the present invention does not require rotation for its insertion and can be seated by linear advancement.
The spinal fusion implant of the present invention is generally effective, and is safer and more effective than the cylindrical implants of the prior art for the special instance when it is desirable to insert two implants side-by-side into cylindrically prepared channels, and where the height of the disc space between two adjacent vertebrae is so great relative to the transverse width of the spine, that two implants of the requisite height will not fit within the transverse width of the spine. Prior art has taught those knowledgeable in the art of spinal surgery, that the likelihood of obtaining a spinal fusion is proportionate to three factors 1) the surface area of the implant 2) the quality and quantity of the graft material and 3) the stability of the fusion construct. The spinal fusion implant of the present invention increases each of these three factors by making it possible to use two implants side-by-side across a disc space that would otherwise lack sufficient width to accept more than one.
The spinal fusion implant of the present invention is more efficacious than the prior art on an individual implant basis for the following reasons:
1. Increased surface area. The spinal fusion implant of the present invention, because of its surface roughenings has greater surface area for engaging the adjacent vertebrae than an implant with smooth external surfaces. The presence or absence of holes does not materially affect this, so far as the holes are filled with material effectively contributing to the area of contact at the surface. The arced portions of the partially cylindrical implant of the present invention are in contact with the adjacent vertebrae and provide a greater surface area than is possible with a flat portion from a non-cylindrical implant.
2. The quantity and quality of graft material presented. As the spinal fusion implant of the present invention is not screwed in, it need not be constructed to resist the torquing therewith associated. Thus, the implant of the present invention may be thinner walled and thereby, for a given diameter, have greater internal volume. The spinal fusion implant, of the present invention has arced portions making the implant stronger in compression than an implant lacking upper and lower curved supporting surfaces such that the wall of the implant can be relatively thinner than such implants. A thinner wall is easier for bone to grow through. Also, the interpore bridges may be smaller allowing for greater porosity and thereby greater exposure to the internal graft material. Further, the spinal fusion implant of the present invention may be constructed of and/or coated with, and/or loaded with a variety of materials and/or chemical substrates known to actively participate in the bone fusion process. As the spinal fusion implant of the present invention offers greater surface area, and greater internal volume for its outside diameter, it offers the opportunity for presenting a greater surface area and volume of these fusion materials.
3. The implant of the present invention offers greater stability than the prior art implants. The least stable implants are the implants lacking surface roughenings. Surface holes increase implant stability by increasing the interference of the implant to the opposed surfaces. The spinal fusion implant of the present invention is a further improvement over the prior art in that the surface roughenings of the spinal fusion implant of the present invention resist motion in all directions. Further, all implants are subject to the possibility of backing out, by retracing the path by which they were inserted. However, the spinal fusion implant of the present invention can have a surface configured to urge the spinal fusion implant forward as to offer increased resistance against such undesirable backward migration. Further, the arced portions of the implant of the present invention provide a greater support area to better distribute the compression forces through the vertebrae.
The spinal fusion implant of the present invention is easier to use as it occupies less space, does not require pre-tapping, and can be inserted without the need to rotate an instrument within the closed confines of thee spinal wound. Further, the spinal fusion implant of the present invention is easier to insert than implants lacking upper and lower curved supporting surfaces that are arcs of the same circle and which implants are to be inserted across the disc space and into the adjacent vertebrae as it is easier to prepare a round hole than a square hole, as a round hole can be drilled in a single step.
It is an object of the present invention to provide an improved interbody spinal fusion implant such that it is possible to place two such implants side-by-side across a disc space and into two adjacent vertebrae in close approximation to each other and within the transverse width of the spine, where the transverse width of the spine would have otherwise been insufficient relative to the required implant height to have allowed for the accommodation of two prior art cylindrical threaded implants.
It is another object of the present invention to provide a spinal fusion implant that is easier to insert, and does not require tapping prior to implantation.
It is yet another object of the present invention to provide a spinal fusion implant that is safer, in which there is no need to run sharp threads near delicate structures.
It is still another object of the present invention to provide a spinal fusion implant that is faster to implant between adjacent vertebrae via linear advancement as opposed to rotational advancement.
It is yet another object of the present invention to provide a method for implanting partially cylindrical implants having at least one flat side.
These and other objects of the present invention will be apparent from a review of the accompanying drawings and the following detailed description of the drawings.